The present invention relates in general to drill bits. More particularly, the invention is directed to a selective coating which improves the performance of microdrills.
Multilayer electronic printed circuit boards require electrical interconnections between the copper patterns in the various layers. These interconnections, commonly called vias or plated through holes, convey the electrical power, electrical ground and processed signals between the circuits defined by the various board layers. The actual connection from layer to layer is accomplished by forming copper plating in holes drilled through the board, the locations of such holes thereby defining the interconnections between the lovers. As electronic devices shrink in size and increase in operating frequency, the individual sizes and numbers of holes per square inch of board surface continue to increase. In conventional practice, the holes which define the locations of the vias are drilled using high speed small diameter drill bits, commonly referred to as microdrills.
The conventional process for making vias in printed circuit boards has two aspects. First, a mechanical process, called drilling, is used to create the holes in the fiberglass/copper laminate of the multilayer board. Thereafter, a chemical process, called plating, forms conductive copper in the mechanically drilled holes. Therefore, the quality of the hole drilling process directly effects the quality of the via formed in the board, in terms of hole location accuracy and hole smoothness.
It is not unusual to have approximately 20 percent of printed circuit board plant capital invested in mechanical drilling process equipment. In relative comparison to other aspects of the printed circuit board manufacturing, the drilling process is slow. Each hole is drilled in succession on a drilling machine which maintains the high degree of precision required for miniaturized multiple layer printed circuit boards. A typical machine drills approximately three holes per second at a nominal chip load.
Printed circuit board industry microdrill bits vary in diameter from approximately 0.1 to 0.5 millimeters. Industry standard twist type microdrill bits are fabricated from 92 percent tungsten carbide crystals cemented with an 8 percent cobalt binder. The tungsten carbide exhibits wear resistance suitable for cutting through laminated printed circuit boards having multiple epoxy, copper and silicon glass layers. In typical usage such microdrills are capable of making approximately 1100 holes before they must be replaced or resharpened. Industry wide the range varies from 1000-2000 holes.
Investigation has confirmed that only the cutting surface of the microdrill requires the toughness provided by the tungsten carbide. The prevailing use of 92 percent tungsten carbide is the result of empirical studies undertaken to reduce the hole formation time by increasing the drill speed and the penetration rate during the drilling process. Attempts to increase the tungsten carbide content of the drill bits, in pursuit of higher drill speeds and better hole location accuracy, has proven unsuccessful because of the effects of the board chips being removed during the drilling process. As the cutting rate is increased, the chips bind in the drill bit flutes, causing rough holes or catastrophic breakage of the drill bits. The latter effect typically results in the scrap of the printed circuit board.
The shrinking dimensions of printed circuit boards also accentuates the importance of hole location accuracy. Conventional microdrills, when used to simultaneously drill holes through three stacked printed circuit boards, provide a 3 sigma location accuracy of approximately 0.075 millimeters. Investigation of microdrill bit compositions show that higher concentrations of tungsten carbide lead to better location accuracy. However, such higher concentrations also result in rougher holes and higher occurrences of drill bit breakage, as described earlier.
Accordingly, what is needed is a microdrill bit design which maintains or improves hole location accuracy, improves hole roughness, allows faster hole formation, and extends the life of microdrill bits between resharpening cycles.